Proper procedure for NPT tapping & threading?

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EOsteam
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Proper procedure for NPT tapping & threading?

Post by EOsteam »

I just dread when I see an NPT thread is required on a project. For instance, last evening a 1/4” NPT thread was required for an air manifold project. The charts specified a 7/16” tap drill followed by the 1/4” NPT tap. The material was Aluminum. Lubricant was used as well as proper technique or what I believe is proper tecnique. Due to the taper on NPT threads the force required gets to be quite high. At one point the tap was only advanced 1/8 of a turn before having to back out 1.5 turns to clear the hand tap. I had purchased a new Starrett 94D tap wrench to make sure I had the proper tooling on hand. Nevertheless, the tap wrench broke where the 90 degree portion engages the tap. What is the proper procedure for NPT threads? Are there special taper drills dedicated to NPT tapping? I’m sure that my lack of knowledge is somehow at fault here. I would most appreciate any and all advice on how to make this a routine procedure in my shop.
Thank you.
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Bill Shields
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Re: Proper procedure for NPT tapping & threading?

Post by Bill Shields »

For the size you are referencing, just a single size drill is used

Most important is to get the tap straight in the hole.

It is a blue moon when I start a tap free hand without having a center in the end of the wrench on a lathe, or the tap guided by a tapping machine, or the tap literally held in the chuck of my vertical milling machine.

Like many non-production machining experiences . It is as much an art as painting a picture.

Aluminum (some soft varieties) is nasty and benefit from aluminum specific lubricant.

Sharp tap? Sand in the aluminum casting dulling the tap?
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GlennW
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Re: Proper procedure for NPT tapping & threading?

Post by GlennW »

Sounds like a very dull (or or hardware store carbon steel) tap, or not properly aligned with the hole. Or possibly both!

There are tapered pipe reamers available to create a tapered hole before tapping, but shouldn't be needed for a 1/4 Pipe tap in aluminum.
Glenn

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Richard_W
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Re: Proper procedure for NPT tapping & threading?

Post by Richard_W »

In soft material I like to use and interrupted pipe tap. It reduces the cutting force and less likely to strip out the threads.

https://www.mscdirect.com/browse/tn?nav ... m=pipe+tap

In the smaller sizes of NPT taps I like the spiral flute taps. The chips will curl out and give better results, because the chips are pushed up out of the way.

https://www.mscdirect.com/browse/tn?nav ... m=pipe+tap

Richard W.
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Steggy
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Re: Proper procedure for NPT tapping & threading?

Post by Steggy »

Richard_W wrote: Sat Dec 31, 2022 2:08 pm In soft material I like to use an interrupted pipe tap.

Ditto. An interrupted thread tap works well in any material, but especially in soft metals such as aluminum and brass. Only downside is the interrupted-thread tap will cost more than its full-thread counterpart.

My go-to lubricant for drilling, milling and tapping aluminum is a 10/90 mixture of petroleum-based cutting oil and mineral spirits. In a pinch, WD-40 will do.
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EOsteam
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Re: Proper procedure for NPT tapping & threading?

Post by EOsteam »

Thank you for all of your replies. I believe we have the culprit. A tap guide was used to center the tap but the tap appeared to be a carbon steel hardware store tap. One of my next orders will be to include a HSS new 1/4-18 NPT tap. That tap should be the last hardware store carbon steel tap in my collection. Again thank you all and Happy New Year!
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Bill Shields
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Re: Proper procedure for NPT tapping & threading?

Post by Bill Shields »

Nothing wrong with a sharp cabon steel tap.

I have boxes full of them that I still use.

You just need to know when to throw them out.
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Harold_V
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Re: Proper procedure for NPT tapping & threading?

Post by Harold_V »

Bill Shields wrote: Mon Jan 02, 2023 8:16 am Nothing wrong with a sharp cabon steel tap.
Key, here, is knowing what "sharp" means, and I fully agree. Carbon steel performs perfectly well when it's sharp and will do so unless temperature elevates to the point where hardness is drawn. The real problem with carbon steel is that it doesn't tolerate heat being generated in the cut. It is more than hard enough to perform as a cutting tool.

Sharp.

Look at the cutting edge of the tap. See any rounded, shiny areas? There should be none. If a tap displays any rounding of the cutting edge, or shiny spots, that's where the tap is no longer cutting, but pushing metal instead. Tapping pressure elevates exponentially in that scenario.

If a cutter grinder is at one's disposal, tapered taps can be successfully resharpened. Doing so by hand risks destroying the required rake angle of the tap.

H
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liveaboard
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Re: Proper procedure for NPT tapping & threading?

Post by liveaboard »

Recently I had to cut a tapered male 1/2" pipe thread; I looked up the taper and cut the blank rod to that spec, then cut the thread by backing out the cross feed during the cut to try and keep it following the taper.

I succeeded; but I didn't need the threads to be the seal (the fitting has a shoulder).

It made me wonder though; what is the right way to do this?
The pipe dies I have require tremendous force and I've never had a good result from them unless I could cut on the lathe first.
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Bill Shields
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Re: Proper procedure for NPT tapping & threading?

Post by Bill Shields »

Taper attachmentbon lathe..?

Thread mill...but that requires a control to drive.
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Harold_V
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Re: Proper procedure for NPT tapping & threading?

Post by Harold_V »

An alternate method is to use a hydraulic duplicator to generate the thread. Doing so requires a finger that has a spring loaded flap that prevents the threading tool from returning to depth when it is retracted for the following cut. The process, otherwise, is no different from creating a straight thread with a lathe using the half nuts.

H
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Howard Gorin
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Re: Proper procedure for NPT tapping & threading?

Post by Howard Gorin »

Nice thing about a carbon steel tap is when you break it, just anneal the tap, heat it red, let it cool slowly and simply drill it out.
You can also dissolve the tap by heating it a solution of water and alum. Keep the solution just below the boiling point, add water
as necessary and you should see a tiny bubble start to form occasionally pick out the disintegrated pieces of the tap. This will work better with carbon steel taps rather than with high speed taps.
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